The basic plot is that Stitch is in confinement ready for teleport, when he manages to break free. The lights go out and he starts 'running' around the theatre, jumping on people, belching at them and playing with their hair.
The harnesses on the ride push down very hard, but if you hunch your shoulders up as high as you can when it's lowering, it won't be so tight. I have heard of people being on there when kids have got so scared that they've flipped out and tried to get out of their harness.
It is dark, and there's a lot of screaming (some real some pre-recorded). If your son isn't good with dark or loud noises, this might not be the best attraction for him. There are times when it's almost pitch black, Stitch sometimes talks right into your ear, and the harness may make him feel that he's trapped in this situation.
That said, if he's really got his heart set on seeing it, you might be able to ask a CM if you and he could stand near the door to watch it. You wouldn't get the full effect, but you would be able to get out quickly if needed. Alternatively someone from your group could sneak away for half an hour to test it first.
In my opinion, you wouldn't be missing too much by avoiding it, but I know a number of people who love it. One warning though, Stitch doesn't become 'nice Stitch' at the end, he just gets captured again and you leave. I know some kids would find it a lot less scary if it had a 'happier' ending.
Good luck, and I hope you have a great time!
